Square wave modulation of a mirror lens for ion isolation in a Fourier transform electrostatic linear ion trap mass spectrometer

Isolation of ions is a critical step in tandem mass spectrometry experiments. In electrostatic linear ion trap mass spectrometers, isolation is typically performed either using a timed ion selector, or by using a periodic electric field orthogonal to the axis of ion motion. In the latter case, the frequency of the field is matched to an ion of interest and the phase is such that the field is zero when the ions of interest are passing through the affected region. The periodic field method has the advantage that it is easier to isolate a single ion from a complex mixture. Here we describe an isolation method that produces essentially identical results to the orthogonal field method, but does not require the addition of any electrodes to the ion path. Instead, the periodic signal (a high voltage square wave) is applied to a lens that is a constituent of one of the ion mirrors. The square wave alters the energy of contaminant ions causing them to be lost from the trap. We demonstrate isolation of ions from a complex mixture using the square wave modulation technique. We also demonstrate an isolation resolution of 200 by isolating isotopes of a small molecule. We also characterize the performance of this method as a function of amplitude and time. An amplitude as low as 200V applied for 1 ms can result in high-quality isolations using this method. We also discuss important considerations for those wishing to implement the method on other instruments. (c) 2014 Elsevier B.V. All rights reserved.